The present invention relates to a method of automatically setting each of frequencies in each of a plurality of wireless microphone-receivers simultaneously used in the same area, based on the combinations of simultaneously usable frequencies. Each of microphone-receivers 10A to 10N is mutual-communicatably connected by a connecting cable CA. One and the other microphone-receivers are designated as a master microphone-receiver 10A and slave microphone-receivers 10B to 10N, respectively. First, an empty channel-information is sent from the master microphone-receiver 10A to a first slave microphone-receiver 10B and the frequency of the empty channel-information is set in the first slave microphone-receiver 10B. The empty channel-information is renewed in the master microphone-receiver. In the same manner frequencies except interfering radio waves are set in number order to the other slave microphone-receivers following the first slave microphone-receiver by mutually communicating with the master microphone-receiver.